This invention relates to improved photographic silver halide emulsions, and to a process for the production of such emulsions.
Silver halide emulsions are composed of silver halide crystals dispersed in a colloid medium which is often gelatin. The properties of the photographic emulsions depend very markedly on the several steps which are used to prepare the photographic emulsion, and the relation and order of one or more of such steps with respect to each of the others.
Thus, a common process for preparation of such an emulsion comprises the initial precipitation (nucleation) of microscopic silver halide crystals, usually by mixing of a silver salt solution with a water-soluble halide salt solution; growth of these crystals by further addition of reagent solutions; washing of the emulsion to remove water-soluble salts formed as a by-product of the double-decomposition reaction of the previous precipitation stages, and sensitisation in order to increase the intrinsic sensitivity of the final emulsion by treatment with chemical sensitising agents such as sulphur and gold salts, and in many cases by the addition of spectral sensitising dyes.
The steps of such an emulsion-making process can be designed precisely to meet various desirable objectives so that emulsions having the requisite photographic properties may be obtained. Thus, the precipitation stages in the process may be adapted to control the average size of the silver halide crystals (which in general determines the speed of the photographic emulsion), the size distribution of those crystals (which affects photographic contrast), the shape and habit of the crystals (including the external lattice faces and the extent of twinning) and the halide composition of the crystals. It is particularly advantageous to control the uniformity of the halide distribution within the population of crystals, and the halide compositional profile within each crystal. The shape and internal structure of the crystals have also an important influence on the photographic behaviour of the emulsion. In particular twinned silver halide crystals are favored in many applications, because of their high photographic sensitivity and covering power (ratio of developed density to weight of developed silver).
This invention relates especially to silver halide crystals which are of particular shape and habit. However in order to achieve this selection of shape and habit it has been found necessary that some restriction of the halide composition is also required. Specifically, this invention relates to an improved method for the production of silver iodobromide, silver iodochloride or silver iodochlorobromide emulsions of the twinned type by a controlled incorporation of silver iodide in the silver halide crystals during growth.
Improved photographic properties are often observed when a mixture of water-soluble halides is used in the precipitation of the silver halide. In general there are two recognised techniques for controlling the precipitation of silver halide, as described by Gutoff in U.S. Pat. No. 3,773,516; the single-jet and the double-jet emulsification methods. In the single-jet process aqueous silver nitrate solution is added to a solution containing a small amount of gelatin and a mixture of soluble halides. The crystal size distribution may be controlled by an Ostwald ripening step after part or all of the silver nitrate has been introduced, in which the emulsion is held at elevated temperatures in the presence of a silver halide solvent. During this step the least soluble, large crystals grow by diffusion and incorporation of silver halide dissolved from the more soluble, small crystals.
The ripening stage results in an increase in average crystal size of the emulsion (and often an increase in the photographic sensitivity of the emulsion ultimately) and a widening of the crystal size distribution. The crystal habit of photographic emulsions made by a single-jet technique often is predominantly that of twinned octahedral crystals, due to the large excess of halide ions present during the precipitation and ripening stages. This is especially true of iodobromide precipitations. A description of twinned crystals is given in "An Introduction to Crystallography", 3rd edition, Longmans (1966) pp. 162-165 by F. C. Phillips and "The Crystalline State", by P. Gay, Oliver and Boyd (1972) pp. 328-338. The disadvantage of such a single-jet process is that the twinned crystals produced invariably have a relatively wide size distribution. In the alternative double-jet process, aqueous solutions of silver nitrate and soluble halide salts are added simultaneously to a stirred solution of gelatin. The flow rates of the reagent solutions may be regulated so that the quantity of excess halide is maintained constant, and predominantly untwinned crystals are formed.
Silver halide crystals of flat or tabular shape exhibit on development extremely good covering power and thus the silver utilisation is extremely good compared with silver halide crystals of other shapes for example cubic crystals. Many twinned octahedral crystals are of this type particularly if the crystals contain more than one twin-plane and the twin planes are parallel. A particular object of the present invention is to increase the proportion of twinned crystals with parallel twin planes in an emulsion.
Another objective of improvements in the commercial production of photographic emulsions is to increase the contrast of the final material, this being a desirable property for graphic arts and radiographic products. This may be achieved partly as a result of a decrease in size distribution, as for example described in British patent specification No. 1,469,480, and partly by ensuring that the iodide content and iodide distribution of different silver halide crystals in the emulsion are made more similar. The point of addition of the soluble iodide salt in various emulsification processes is known to affect the sensitivity and size distribution of the emulsion (Research Disclosure No. 13,452 (1975)).
Thus the application of photographic emulsions containing twinned crystals to products requiring enhanced sensitivity and contrast has been hindered by the defects in conventional emulsification procedures particularly the uncontrolled incorporation of iodide in the crystals. There was described in copending in U.S. Pat. Application Ser. No. 799,040, filed May 20, 1977, now U.S. Pat. No. 4,150,994, an improved method for the preparation of silver iodobromide, silver iodochloride or silver iodochlorobromide photographic emulsions, wherein iodide ions are supplied to the growing crystals by the dissolution of a uniform dispersion of silver iodide crystals. There was also described the conditions under which the desired tabular twinned habit is favoured by this process, and by which substantially uniform photographic emulsion crystals of this type may be prepared.
Thus there is described in said copending application a method of preparing a silver halide emulsion of the twinned type which comprises the steps of (a) forming in a colloid dispersing medium silver halide crystals containing at least 90 mole % iodide, (b) mixing in the dispersing medium containing the said silver halide crystals an aqueous solution of a silver salt and an aqueous solution of an alkali metal or ammonium bromide or chloride or mixtures thereof so forming twinned silver halide crystals containing iodide and the halide or halides being added, (c) adding a silver halide solvent to the dispersing medium and so causing the growth of the twinned crystals by Ostwald ripening, optionally (d) causing the twinned crystals to increase in size by adding to the colloidal dispersing further aqueous silver salt solution and further alkali metall or ammonium halide and then finally optionally (e) removing the water-soluble salts formed and chemically sensitising the emulsion.
It has now been discovered that the process as just described can be modified by chemically sensitising the silver halide crystals containing at least 90 mole % iodide in step (a).